The present invention generally relates to electrography and more particularly, to a method of developing electrostatic latent images carried on the surface of an electrostatic latent image support member, through employment of developing sleeves having magnetic rollers rotatably accommodated therein.
As one form of the developing apparatus for effecting the developing method as referred to above, there has been conventionally proposed an arrangement, as shown in FIG. 1, in which a magnetic developing material De composed of a mixture of magnetic carrier and electrically insulative toner is supplied onto the peripheral surface of a developing sleeve S, while a magnetic roller M provided within the developing sleeve S is driven for rotation in a direction indicated by an arrow m so as to transport the developing material De in the direction of an arrow t along the peripheral surface of the developing sleeve S for rubbing a photoconductive surface Pa of a photosensitive or photoreceptor drum P being driven for rotation in a direction of an arrow a, by the developing material De thus transported, thereby to develop the electrostatic latent image carried on the surface Pa of the photoreceptor drum P into a visible image.
In the developing apparatus as described above, it has been experienced that, in the case where the number of revolutions of the magnetic roller M is set at a small value, the frequency of an alternating magnetic field to be formed in a developing region is also lowered to a small value, and the developing itself of the electrostatic latent image is heavily affected by the alternating magnetic field, thus resulting in a periodic developing irregularities or uneven developing as shown in FIG. 2, which schematically illustrates a developed image as obtained by developing a solid state electrostatic latent image under a state where the number of revolutions of the magnetic roller M is set at a small value, and in which the arrow a indicates the direction of movement of the photoreceptor drum P with respect to the developing apparatus.
In connection with the above, it has been ensured by the present inventor that the frequency Fmg of the alternating magnetic field may be represented by a following equation EQU Fmg=Nmg.multidot.Rmg/60 (1)
where Nmg is the number of poles of the magnetic roller and Rmg represents the number of revolutions (r.p.m.) of the magnetic roller, while a pitch w of the developing irregularities, i.e. the distance between respective dark and light portions in the developed image as shown in FIG. 2 coincides with the value represented by an equation given below EQU w=Vpc/Fmg (2)
where w is the pitch (mm) of the developing irregularities and Vpc represents a circumferential speed (mm/sec.) of the photoreceptor drum P.
Meanwhile, upon investigation into formation of developing irregularities through variations of the circumferential speed of the photoreceptor drum P and the number of revolutions of the magnetic roller M in various ways, it has been noticed that the developing irregularities tend to take place in a region below a straight line shown in a graph of FIG. 3, while the formation of such developing irregularities is not noticed in a region above the straight line.
In the above experiment, the developing sleeve S having a diameter of 31 mm was driven for rotation at 25 r.p.m. at all times in the direction indicated by the arrow d in FIG. 1, with the number of poles of the magnetic roller M being set to be eight poles. However, since the amount of the developing material De transported towards a developing region is altered, upon variations of the diameter or number of revolutions, etc. of the developing sleeve S, above the region at which the actual developing irregularities become conspicuous, is consequently altered.
As is seen from the graph in FIG. 3, it is possible to prevent formation of developing irregularities by setting the number of revolutions of the magnetic roller M at a sufficiently large valve, and with respect to the developing apparatus of this kind, high speed driving of the magnetic roller M is required for obtaining a uniform developed image without developing irregularities. Meanwhile, the tendency to form developing irregularities is strengthened, directly depending on the increase of circumferential speed of the photoreceptor drum P.
Therefore, in the case where the circumferential speed of the photoreceptor drum P is increased for a higher copying speed in an electrographic copying apparatus incorporated with the developing apparatus as described above, it is required to increase the rotational speed of the magnetic roller M also by taking into account, the formation of developing irregularities as stated above. By way of example, as is clear from the graph of FIG. 3, in the case where the circumferential speed of the photoreceptor drum P is set at 200 mm/sec. the number of revolutions of the magnetic roller M in the order of 1000 r.p.m or thereabout is sufficient for preventing formation of the developing irregularities, while upon increase of the circumferential speed of the drum P up to 300 mm/sec., the number of revolutions of the magnetic roller M should be at least higher than 1600 r.p.m.
On the other hand, with respect to the developing apparatus as described so far, generation of various inconveniences or troubles following high speed driving of the magnetic roller M has also been experienced. More specifically, as the magnetic roller M is driven for rotation at high speeds, there has been experienced generation of inconveniences due to heat radiation and eddy current loss in which energy is lost in the form of heat through production of a large amount of eddy current in the developing sleeve S, electrically conductive members located in the vicinity thereof or in the electrically conductive layer Pa of the photoreceptor drum P, or generation of inconveniences in which the developing apparatus is subjected to slight vibrations arising from a large torque applied thereto during the high speed driving of the magnetic roller M. The degree of generation of such inconveniences as described above tends to be cumulatively increased as the number of revolutions of the magnetic roller M is raised, and therefore, with respect to the developing apparatus as described so far, while the high speed driving of the magnetic roller M is required for the prevention of generation of the developing irregularities, reduction in the driving speed of the magnetic roller M is simultaneously required as a requirement contrary to the above.
Such being the case, in balancing one requirement with the other as referred to above, the developing apparatus described so far is not suited to the development of electrostatic latent images carried on the surface of the electrostatic latent image support member which is being driven at high speeds, while, in order to obtain a uniform developed image without developing irregularities, generation of the inconveniences as described earlier must be admitted to a certain extent under the present situation.
Furthermore, with respect to the developing material recently proposed by the present inventor, i.e. the developing material composed of a mixture of electrically insulative toner having average particle diameter of 5 to 20 .mu.m and high resistance magnetic carrier prepared by dispersing magnetic fine particles into the resin so as to have average particle diameter of 15 to 60 .mu.m and resistance value of 10.sup.12 .OMEGA.cm in the case of developing electrostatic latent images through employment of the developing apparatus as described so far, high speed rotation of the magnetic roller M is required for obtaining developed images at high image density.
Therefore, for developing the electrostatic latent images through employment of the developing material as described above, the problems referred to earlier may also be brought about in association with the achievement of developed images having a sufficient image density, apart from the high speed driving of the electrostatic latent image support member.